1. Field of the Invention
The present invention relates to laser systems useful for precision linear and planar alignment, and more particularly, to an apparatus that performs a light energy centroid measurement between a pair of reference laser beams in order to significantly improve alignment accuracy.
2. Description of Related Art
Laser emitters and detectors are commonly used in industry to perform precision alignment and measurement. In one type of such system, a laser device emits a laser beam that can provide a relatively accurate reference line. A detector operated in conjunction with the laser can measure displacement between the beam and an object requiring alignment to the laser axis.
Alternatively, a laser device may be oriented with a rotating pentaprism or penta-mirror assembly that deflects the laser beam through a precise 90.degree. angle and sweeps the beam to provide a flat plane that is perpendicular to the input laser beam. The plane can be used as a reference to which the alignment and/or position of other objects can be compared. Such laser alignment systems have various applications within construction, surveying or manufacturing. In one possible application, a laser beam alignment device could be used in commercial construction to define a plumb line or a planar laser beam alignment device used to build a wall. Examples of such laser beam alignment devices are disclosed in U.S. Pat. No. 4,676,598 to Markley et al. for MULTIPLE REFERENCE LASER BEAM APPARATUS, and in U.S. Pat. No. 4,662,707 to Teach et al. for LIGHTHOUSE STRUCTURE AND COMPENSATING LENS IN REFERENCE LASER BEAM PROJECTING APPARATUS.
Traditionally, the laser beam in such alignment systems is generated by a laser tube containing a suitable active gaseous element, such as helium-neon gas. The gas is excited by an optical or electrical source to emit a low-powered collimated output beam in the red band of the light spectrum. These so-called gas lasers produce a thermally stable beam that is useful for producing highly accurate measurements over relatively long distances (e.g., hundredths of an inch over one hundred feet of distance). Despite the inherent stability of reference beams produced by gas lasers, however, these laser devices are also very energy inefficient, expensive, relatively large and fragile.
In view of the noted deficiencies of gas lasers, laser diodes have increased in commercial popularity as a suitable alternative. A laser diode is a semiconductor device, usually of the gallium-arsenide type, that emits coherent light when a voltage is applied to its terminals. Laser diodes are substantially less expensive than gas lasers, and can be manufactured in a smaller, rugged, more compact package. A significant drawback of laser diodes is that they produce a far less stable beam than gas lasers. In particular, the laser beam does not have uniform intensity over its cross-section and the energy of the beam decreases with distance from the center of the beam. Moreover, the rate of change of the intensity is not uniform, and the center of beam energy will periodically shift in accordance with temperature. As a result of these deficiencies, a linear or planar alignment device using a laser diode cannot be expected to achieve the same degree of accuracy as gas lasers.
Prior art devices have sought to overcome the inadequacy of laser diodes in order to approximate the stability of gas lasers. In particular, U.S. Pat. No. 5,307,368 to Hamar for LASER APPARATUS FOR SIMULTANEOUSLY GENERATING MUTUALLY PERPENDICULAR PLANES discloses a laser alignment device having a hollow spindle with an aperture that is rotatably mounted within bearings. Laser light from a laser diode floods the aperture so that only the centermost portion of the beam passes entirely through the spindle. The laser alignment device disclosed by Hamar purports to provide a centered beam of uniform density and circular cross-section. In practice, however, the resulting beam still cannot achieve the stability of gas laser beams due to thermal shifts over time within the center of beam energy that passes through the spindle.
Notwithstanding this significant drawback, laser diodes are still acceptable for most alignment applications. In general, commercial construction applications do not require a level of accuracy high enough to justify the additional expense of a gas laser, and, in these applications, a laser beam generated by a laser diode can provide a sufficient level of stability. As a result, demand for laser diodes has totally outstripped demand for gas lasers and, currently, there are few suppliers willing to produce gas lasers to satisfy the particular applications that require greater accuracy than that achievable with laser diodes.
Accordingly, a critical need exists for a laser alignment apparatus capable of providing a highly stable and accurate beam suitable for such critical linear and planar alignment applications. Such a laser alignment apparatus should be capable of providing a level of accuracy heretofore achievable only with gas lasers, while maintaining the economical attributes of commercial laser diodes.